Glass composition fluorescent at infrared wavelengths

ABSTRACT

The present invention provides a glass composition that exhibits a fluorescence function and an optical amplification function in a wide wavelength range. This glass composition includes a bismuth oxide, an aluminum oxide, and a glass network former. The glass network former includes an oxide other than silicon oxides as its main component. The glass composition emits fluorescence in an infrared wavelength region through irradiation of excitation light, with bismuth contained in the bismuth oxide functioning as a fluorescent source. A preferable glass network former is B 2 O 3  or P 2 O 5 . This glass composition further may contain a univalent or divalent metal oxide.

TECHNICAL FIELD

The present invention relates to a glass composition that can functionas a light emitter or an optical amplification medium.

BACKGROUND ART

Glass that includes a rare earth element such as Nd, Er, Pr, etc. andemits fluorescence in the infrared region has been known. Laser emissionand optical amplification that were achieved using this glass werestudied mainly in the 1990s. Fluorescence of this glass is caused byradiative transition of the 4f electron of a rare earth ion. Since the4f electron is covered with an outer-shell electron, the fluorescencecan be obtained only in a narrow wavelength region. This limits theranges of the wavelengths of light that can be amplified and thewavelengths at which laser oscillation can occur.

With consideration given to this, each of JP11(1999)-317561A andJP2001-213636A discloses a glass composition that includes a largeamount (for instance, at least 20 mol %) of Bi₂O₃ as well as Er as afluorescent element and that allows a wavelength range of 80 nm orlonger to be used. However, since the fluorescent source is Er, theextension of the wavelength range is limited to about 100 nm. Inaddition, the refractive index of the glass composition is as high asabout 2. Accordingly, when it is connected to a silica glass opticalfiber that is used in optical communications, a problem tends to becaused by reflection at the interface therebetween.

Each of JP6(1994)-296058A, JP2000-53442A, and JP2000-302477A discloses aglass composition that includes Cr or Ni as a fluorescent element andallows fluorescence to occur in a wide wavelength range. In the glasscomposition including Cr as a fluorescent element, its main component isAl₂O₃ and its glass network former is limited to a small amount (20 mol% or less). Accordingly, this glass composition tends to devitrify whenbeing melted or formed. It is necessary for the glass compositionincluding Ni as a fluorescent element to contain at least one of a Ni⁺ion, a microcrystal including a Ni²⁺ ion, and a Ni ion having ahexacoordinated structure. In addition, fine particles of Ni deposit.Accordingly, this glass composition also tends to devitrify.

JP11(1999)-29334A discloses a silica glass doped with Bi. In this glasscomposition, Bi has been clustered in zeolite and thereby fluorescenceis obtained over an increased wavelength range. In this silica glass,however, Bi has been clustered and therefore respective Bi elements areextremely close to each other. Hence, deactivation tends to occurbetween adjacent Bi elements, which results in lower efficiency inoptical amplification. Since this silica glass is produced using asol-gel method, the occurrences of shrinkage during drying and cracksduring baking are problems in mass production of large-sized glass oroptical fibers.

JP2002-252397A discloses an optical fiber amplifier includingBi₂O₃—Al₂O₃—SiO₂ silica glass. With this, amplification of light in the1.3-μm range can be carried out using a 0.8-μm-range semiconductor laseras an excitation light source. This amplifier is excellent incompatibility with silica glass optical fibers. It, however, isnecessary to melt the silica glass at 1750° C. or higher and it has adeformation point of at least 1000° C. Accordingly, the optical fiberscannot be manufactured readily. Even if manufactured, they have a lowertransmittance.

DISCLOSURE OF THE INVENTION

The present invention is intended to provide a new glass compositionthat exhibits a fluorescence function and an optical amplificationfunction in the infrared wavelength region, particularly in a widewavelength range that is used in optical communications.

A glass composition of the present invention includes a bismuth oxide,an aluminum oxide, and a glass network former. The glass network formercontains an oxide other than a silicon oxide as its main component. Theglass composition emits fluorescence in the infrared wavelength regionthrough irradiation of excitation light, with bismuth contained in thebismuth oxide functioning as a fluorescent source.

In the present specification, the “main component” denotes a componentwhose content by percentage is the highest.

The present invention can provide a glass composition that emitsfluorescence in a wide wavelength range within the infrared region andmelts at a lower temperature than that at which a silica glass melts.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of a light amplifier according tothe present invention that was used as an optical system for evaluatingoptical amplification characteristics.

FIG. 2 is a diagram showing a system for detecting light in the 1100-nmrange, which is included in the optical system for evaluating opticalamplification characteristics.

FIG. 3 is a diagram showing a system for detecting light in the 1300-nmrange, which is included in the optical system for evaluating opticalamplification characteristics.

FIG. 4 is a diagram showing another example of a light amplifieraccording to the present invention that was used as an optical systemfor evaluating optical amplification characteristics of optical fibers.

FIG. 5 is a graph showing examples of light transmission spectra ofglass compositions according to the present invention.

FIG. 6 is a graph showing an example of measurement of the half-heightwidth of the optical absorption peak in a glass composition of thepresent invention.

FIG. 7 is a graph showing examples of fluorescence spectra obtained in aglass composition of the present invention.

FIG. 8 is a graph showing other examples of light transmission spectraof glass compositions according to the present invention.

FIG. 9 is a graph showing further examples of fluorescence spectraobtained in a glass composition of the present invention.

FIG. 10 is a graph showing an example of optical amplificationcharacteristics of a glass composition according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the unit “%” that is used for indicating a compositiondenotes “mol %” in every case.

The glass composition of the present invention includes, as essentialelements, a bismuth oxide, an aluminum oxide (Al₂O₃), and a glassnetwork former. Al₂O₃ is too poor in glass network forming ability to beclassified as the glass network former. A typical glass network formeris a silicon oxide. The present invention, however, employs an oxideother than silicon oxide as the main component of the glass networkformer. This main component is, for instance, a boron oxide (B₂O₃), aphosphorus pentoxide (P₂O₅), a germanium oxide (GeO₂), or a telluriumdioxide (TeO₂), preferably B₂O₃ or P₂O₅. This glass composition can havea deformation point of 750° C. or lower.

Preferably, the glass composition of the present invention has anoptical absorption peak in the wavelength range of 400 nm to 900 nm,preferably 400 nm to 850 nm. It is advantageous that the opticalabsorption peak appears in at least one selected from the wavelengthrange of 400 nm to 550 nm and the wavelength range of 650 nm to 750 nm,preferably in both the wavelength ranges. The optical absorption peakmay appear in the wavelength range of 750 nm to 900 nm.

The wavelength at which the maximum intensity of the fluorescence thatis emitted when the glass composition of the present invention isirradiated with excitation light having a wavelength in a range of 400nm to 900 nm is obtained is in the range of, for instance, 900 nm to1600 nm, preferably 1000 nm to 1600 nm, and more preferably 1000 nm to1400 nm. The present invention allows the half-height width with respectto the wavelength of that fluorescence to increase to at least 150 nm,for instance, to 150 nm to 400 nm. At least the fact that thefluorescent source is a positive ion of bismuth contributes to thisincrease in the half-height width. The glass composition of the presentinvention also can be used as an optical amplification medium thatprovides an amplification gain in at least a part of the wavelengthrange of 900 nm to 1600 nm through irradiation of excitation light.

Preferably, the glass composition of the present invention furtherincludes a univalent or divalent metal oxide. This oxide facilitatesvitrification. A suitable divalent metal oxide is at least one selectedfrom MgO, CaO, SrO, BaO, and ZnO. A suitable univalent metal oxide is atleast one selected from Li₂O, Na₂O, and K₂O. MgO and Li₂O are preferablecomponents. It therefore is preferable that the glass compositioninclude at least one of the two oxides. A suitable content by percentageof the univalent or divalent metal oxide is 3% to 40%.

In the glass composition of the present invention, it is preferable thatthe content by percentage of bismuth oxide be in the range of 0.01% to15%, particularly 0.01% to 5%, in terms of Bi₂O₃. Preferably, thecontent by percentage of aluminum oxide is 5% to 30%. It also ispreferable that the content by percentage of the main component of theglass network former be 30% to 90%.

Preferable compositions of the glass composition of the presentinvention are described below as examples.

A first example is a composition including B₂O₃ as the main component ofthe glass network former. This composition includes the followingcomponents: 30% to 90% B₂O₃; 5% to 30% Al₂O₃; 0% to 30% Li₂O; 0% to 15%Na₂O; 0% to 5% K₂O; 0% to 40% MgO; 0% to 30% CaO; 0% to 5% SrO; 0% to 5%BaO; 0% to 25% ZnO; 0% to 10% TiO₂; and 0% to 5% ZrO₂, wherein the totalof MgO+CaO+SrO+BaO+ZnO+Li₂O+Na₂O+K₂O is in the range of 3% to 40%, andthe content by percentage of bismuth oxide is 0.01% to 15% in terms ofBi₂O₃.

A second example is a composition including P₂O₅ as the main componentof the glass network former. This composition includes the followingcomponents: 50% to 80% P₂O₅; 5% to 30% Al₂O₃; 0% to 30% Li₂O; 0% to 15%Na₂O; 0% to 5% K₂O; 0% to 40% MgO; 0% to 30% CaO; 0% to 15% SrO; 0% to15% BaO; 0% to 15% ZnO; 0% to 10% TiO₂; 0% to 5% ZrO₂; and 0% to 20%SiO₂, wherein the total of MgO+CaO+SrO+BaO+ZnO+Li₂O+Na₂O+K₂O is in therange of 3% to 40%, and the content by percentage of bismuth oxide is0.01% to 15% in terms of Bi₂O₃. In this example, it is more preferablethat the contents by percentage of SrO and BaO each be 0% to 5%.

An increase in the ratio of salts, for instance, carbonate and ammoniumsalt that are contained in the raw material of the glass composition maycause the raw material to bubble intensely during melting. Theoccurrence of intense bubbling is not preferable in terms oftransparency of glass. Ammonium salt often is contained in the rawmaterial of the glass composition that contains P₂O₅ as the maincomponent of the glass network former. In this raw material, the ratioof ammonium salt is higher. In this case, it is particularly preferablethat the raw material be melted after the ammonium salt is decomposed.

As described above, it is preferable that a glass composition of thepresent invention be manufactured by a manufacturing method thatincludes a melting process in which a raw material of the glasscomposition is melted and a process for cooling the raw material thathas been melted, and the method further include, before the meltingprocess, a heat treatment process in which a first material thatcontains ammonium salt and that is at least a part of the raw materialis maintained at a temperature at which at least the ammonium saltdecomposes.

Examples of phosphorus-containing ammonium salt to be used as the rawmaterial of P₂O₅ include ammonium phosphate, diammonium hydrogenphosphate, and ammonium dihydrogen phosphate. The above-mentioned firstmaterial may contain other salts, for instance, carbonate in addition tothe ammonium salt. Since raw materials that are oxides are not requiredto be heat-treated, they may be prepared as a second material separatefrom the first material. In the heat treatment process, it is preferablethat the material containing the ammonium salt be heat-treated at atemperature of at least 300° C., for instance 500° C. to 1100° C., for asufficient period for decomposing the ammonium salt. The heatingtemperature to be employed in the melting process is at least thetreatment temperature that is employed in the heat treatment process,for instance, 1250° C. to 1500° C.

When bismuth is reduced due to the decomposition of the ammonium salt,the fluorescence function of the glass composition deteriorates. Ittherefore is advantageous that the raw material of bismuth oxide isincluded in the second material that is prepared separate from the firstmaterial. Preferably, the above-mentioned manufacturing method furtherincludes, after the heat treatment process but before the meltingprocess, a process of mixing the first material with a second materialthat includes a raw material of bismuth oxide or a bismuth oxide itself.

In order to prevent bismuth from being reduced, sulfate or nitrate maybe contained as a part of the raw material of glass. Preferably, the rawmaterial of bismuth oxide or the bismuth oxide is allowed to melttogether with at least one selected from sulfate and nitrate.

The following description is directed to methods of evaluatingcharacteristics of glass compositions according to specific embodimentsof the present invention.

Light Transmission Spectrum

A glass sample was cut out and then was polished to have mirror-finishedsurfaces and to be a flat sheet with a size of 20 mm×30 mm×3 mm(thickness) whose respective opposing surfaces were in parallel witheach other. Thus a sheet sample was produced. The light transmissionspectrum of this sheet sample was measured in the wavelength range of290 nm to 2500 nm using a commercial spectrophotometer. It also waschecked whether the optical absorption peak appeared in the respectivewavelength ranges of 400 nm to 550 nm and 650 nm to 750 nm in the lighttransmission spectrum.

The half-height width of an optical absorption spectrum was determinedas follows. First, the light transmission spectrum was converted intothe molar optical absorption coefficient (that is, with the bismuthoxide indicated in terms of Bi₂O₃, the light transmission spectrum wasconverted into the optical absorption coefficient that was obtained when1% of Bi₂O₃ is contained and the optical path has a length of 1 cm).Thus an optical absorption spectrum was prepared. A common tangent totails of both sides of the peak of this optical absorption spectrum wasdrawn, which was used as a base line. A top line then was drawn so as tobe in parallel with the base line and tangential to the peak. Further, amiddle line was drawn that equally divided the distance between the topline and the base line and that was in parallel with those lines. Thedifference in wavelength between two intersections of the middle lineand the spectrum was taken as the half-height width.

Preferably, the light transmission spectrum has an optical absorptionpeak at which the difference between the top line and the base line isat least 0.01 cm⁻¹mol⁻¹, in the predetermined wavelength range.

Fluorescence Spectrum

With a sheet sample identical to that used in the above, thefluorescence spectrum was measured with a commercial fluorescencespectrophotometer. With respect to each excitation light having apredetermined wavelength, the measurement was carried out in thefluorescence wavelength range of 800 nm to 1600 nm. The sample had atemperature equal to room temperature during the measurement.

The following were determined: the wavelength at which the fluorescencepeak appeared in the fluorescence spectrum measured above; thewavelength range (a half-height width of fluorescence) in which theemission intensity was at least half the peak value; and the emissionintensity at the wavelength at which the fluorescence peak appeared. Theemission intensity is indicated with an arbitrary unit. However, sincethe sample shape and the position where the sample is placed during themeasurement are not changed, a comparison in emission intensity can bemade. The half-height width of fluorescence was determined by the samemethod as that used for determining the half-height width of the opticalabsorption peak.

Lifetime of Fluorescence

With a sheet sample identical to that used above, the lifetime offluorescence also was measured with a fluorescence spectrophotometer.The fluorescence decay caused with the passage of time throughexcitation carried out with pulsed light having a predeterminedwavelength was measured. This measurement was carried out at apredetermined wavelength according to the excitation wavelength, forinstance at 1140 nm when the excitation wavelength was 500 nm. A decaycurve thus obtained was subjected to exponential fitting and thus thelifetime of fluorescence was determined.

Optical Amplification Characteristics

The optical amplification characteristics were determined using themeasuring apparatus shown in FIG. 1. The wavelength of excitation lightto serve as an energy source for amplifying light was 532 nm while twowavelengths of 1064 nm and 1314 nm were employed as the wavelength ofsignal light to be amplified. In this apparatus, the excitation lightand the signal light are superposed spatially on each other in the glasssample and thereby the signal light transmitted through the glass sampleis amplified.

A Nd-YAG green laser to be excited with a semiconductor laser (LD) wasused for a light source 26 of excitation light 20 with a wavelength of532 nm and continuous light emitted therefrom was used as the excitationlight 20. The excitation light 20 was focused through a convex lens 52whose focal length was 300 mm. The position of the lens 52 was adjusted,for example, so that the focal point 62 falls on the midpoint of a glasssample 10 in the direction of its thickness.

On the other hand, when signal light 30 with a wavelength of 1064 nm wasused, a Nd-YAG laser to be excited with a semiconductor laser 36 otherthan the excitation light source 26 was employed as a light source andthe signal light 30 was pulsed light with a pulse width ns. When thesignal light 30 with a wavelength of 1314 nm was used, it was continuouslight emitted from a semiconductor laser 36 with that wavelength. Thesignal light 30 was allowed to enter the glass sample 10 from thedirection opposite to that from which the excitation light 20 enteredit. The signal light 30 was focused through a convex lens 54 whose focallength was 500 mm or 1000 mm. The position of the lens 54 was adjustedso that the focal point 62 falls on the midpoint of the glass sample 10in the direction of its thickness. The combination of the focal lengthof the lens 52 and that of the lens 54 was selected so that the areathrough which the signal light beam passed was included well in the areathrough which the excitation light beam passed.

The signal light 30 and the excitation light 20 weremultiplexed/demultiplexed with wavelength selection reflectors 72 and74. These reflectors 72 and 74 were configured so as to transmit theexcitation light 20 but reflect the signal light 30.

When the wavelength of the signal light was 1064 nm, a commontransparent glass sheet was used as the reflector for the signal light.A transparent glass sheet causes a reflection of several % at itssurface. The signal light 30 with a wavelength of 1064 nm emitted fromthe light source (Nd-YAG laser) 36 is reflected partly by the reflector74 and the rest enters the glass sample 10. The signal light 32 that haspassed through it, i.e. the signal light 32 that has been amplified, isreflected partly by the reflector 72 to be led to a photodetectionsystem 80 through a lens 56.

The two reflectors 72 and 74 do not have high reflectance with respectto light with a wavelength of 1064 nm. The signal light 30, however, ispulsed light and therefore has a very large peak value (a megawatt levelat the point from which a laser is emitted). Accordingly, themeasurement thereof is easy. The excitation light 20 passes through thereflector 72 with almost no loss to reach the glass sample 10. Theexcitation light 22 that has not contributed to the opticalamplification in the glass sample reaches the reflector 74. However,since a small quantity of light is reflected by that reflector, noharmful effect is imposed on the signal light source 36.

The photodetection system 80 that is used when the signal light has awavelength of 1064 nm is shown in detail in FIG. 2. The signal light 32led to the photodetection system 80 covered with a shielding cover 88passes through a visible-light cut-off filter 82 and then passes throughan interference filter 84 that allows only light with a wavelength of1064 nm to transmit therethrough to remove light components other thanthe signal light component. The signal light is converted in aphotodetector 86 into an electric signal that corresponds to the lightsignal intensity and then is displayed on an oscilloscope 90 through asignal cable 92. The photodetector 86 to be used herein may be, forinstance, a Si based photodiode.

When the signal light with a wavelength of 1314 nm was used, dielectricmultilayer mirrors with a high reflectance with respect to thewavelength 1314 nm were used as the reflectors 72 and 74. The signallight 30 emitted from the signal light source (LD) 36 with a wavelengthof 1314 nm is reflected by the reflector 74 to enter the glass sample10. The signal light 32 that has been amplified is reflected by thereflector 72 to be led to the photodetection system 80. The excitationlight 20 passes through the reflector 72 with almost no losses to reachthe glass sample 10. The excitation light 22 that has not contributed tothe optical amplification reaches the reflector 74 to be reflectedslightly. In order to prevent that reflected light from entering thesignal light source 36, a dielectric multilayer mirror (not shown in thefigure) was inserted that was configured to have a high reflectance withrespect to a wavelength of 532 nm.

The photodetection system 80 to be employed when the signal light has awavelength of 1314 nm is shown in detail in FIG. 3. The signal light 32led to the photodetection system 80 is focused on a point near a pinhole83 through a lens 58 having a long focal length (for instance, 1000 mm).When the signal light 32 is allowed to pass through the pinhole, itscomponents that travel in directions other than that in which the signallight should travel, i.e. amplified spontaneous emission (ASE) light andscattered light components can be removed. Furthermore, when the signallight 32 is allowed to pass through a prism 55, an excitation lightcomponent with a wavelength of 532 nm is removed and thereby the signallight component alone enters the photodetector 86. The light signal isconverted into an electric signal that corresponds thereto and then isdisplayed on the oscilloscope through the signal cable 92. Thephotodetector 86 to be used herein can be, for instance, a Gephotodiode.

In the optical system shown in FIG. 1, the excitation light 20 and thesignal light 30 travel in the directions opposite to each other.However, the directions in which they travel are not limited thereto.For instance, both the lights may travel in the same direction. Theglass sample may be of not a block-like shape but a fiber-like shape.

The optical amplification carried out using the above-mentioned opticalsystem was measured as follows.

A glass sample 10 was polished to have mirror-finished surfaces thatwere in parallel with each other. Thus a block sample was produced. Thethickness of the glass sample was determined so that the glass samplehad a transmittance of about 95% with respect to the wavelength ofexcitation light, for instance, a wavelength of 523 nm. This glasssample was set in the position shown in FIG. 1 and some adjustments weremade so as to allow the signal light 30 and the excitation light 20 tobe superposed well on each other inside the glass sample 10.

Thereafter, the glass sample 10 was irradiated with the signal light 30and then the intensity of the signal light 32 that had passed throughthe glass sample 10 was measured with the oscilloscope 90. Subsequently,the glass sample 10 was irradiated with the excitation light 20 whilethe irradiation of the signal light 30 was continued, and then theintensity of the signal light 32 was measured with the oscilloscope 90in the same manner as above. The optical amplification phenomenon can bechecked through a comparison that is made between the intensity of thesignal light transmitted during the irradiation of the signal lightalone and that of the signal light transmitted during the simultaneousirradiation of the signal light and the excitation light.

Optical Fiber Amplification Test

The optical amplification characteristics of an optical fiber samplewere determined using the measuring apparatus shown in FIG. 4. Thewavelength of excitation light 21 to serve as an energy source foramplifying light was 808 nm while the wavelength of signal light 30 tobe amplified was 1314 nm. In this apparatus, the excitation light 21 andthe signal light 30 are superposed spatially on each other in thevicinity of an optical fiber end 14 that is a part from which lightenters the core of the fiber sample. Thus the signal light 34 that haspassed through the fiber sample 12 is amplified.

Continuous light emitted from a semiconductor laser was used for each oflight sources 28 and 38 for the excitation light with a wavelength of808 nm and the signal light with a wavelength of 1314 nm.

The signal light and the excitation light were multiplexed/demultiplexedusing a wavelength selection reflector 76. This reflector 76 wasconfigured so as to transmit the signal light 30 but reflect theexcitation light 21.

The light that had come out from the optical fiber 12 was led to aphotodetector 87 through a lens 57. A filter 81 that transmitted thesignal light but intercepted the excitation light was inserted in aplace on the optical path. This allowed only the signal light to bedetected by the photodetector.

In the optical system shown in FIG. 4, the excitation light and thesignal light travel in the same direction, which however is not limitedthereto. For instance, they may travel in the directions opposite toeach other. The wavelength selection reflector may reflect the signallight but transmit the excitation light. Furthermore, the signal lightand the excitation light may be allowed to enter the optical fiber witha means other than the reflector.

The optical amplification carried out using the above-mentioned opticalsystem was measured as follows. The optical fiber sample was cut out tohave sections that were specular surfaces. This was set in theabove-mentioned measuring apparatus. Some adjustments then were made soas to allow the signal light and the excitation light to enter the coreof the optical fiber well.

Thereafter, the end 14 of the optical fiber sample 12 was irradiatedwith the signal light 30 and then the intensity of the signal light 34that had passed through the optical fiber sample 12 was measured withthe oscilloscope 90. Subsequently, the optical fiber sample 12 wasirradiated with the excitation light 21 while the irradiation of thesignal light 30 was continued, and then the intensity of the signallight 34 was measured with the oscilloscope 90. The opticalamplification phenomenon can be checked through a comparison that ismade between the intensity of the signal light transmitted during theirradiation of the signal light alone and that of the signal lighttransmitted during the simultaneous irradiation of the signal light andthe excitation light.

The apparatuses shown in FIGS. 1 and 4, particularly the apparatus shownin FIG. 4, are an example of evaluation apparatus as well as aconfiguration example of a light amplifier according to the presentinvention. As shown in the figures, the light amplifier includes lightsources of excitation light and signal light in addition to a glasscomposition of the present invention. The configuration of the lightamplifier is not limited to those shown in the figures. For instance, asignal-input optical fiber and a signal-output optical fiber may bedisposed instead of the light source of the signal light and thephotodetector, respectively. In addition, the excitation light and thesignal light may be multiplexed/demultiplexed using a fiber coupler. Theuse of such a light amplifier makes it possible to carry out a signallight amplification method in which excitation light and signal lightare allowed to enter a glass composition of the present invention andthereby the signal light is amplified.

Hereinafter, the present invention is described further in detail usingexamples and comparative examples.

EXAMPLE 1 Borate Glass

Commercially available boron oxide, alumina, lithium carbonate, sodiumcarbonate, potassium carbonate, magnesium oxide, calcium carbonate,strontium carbonate, barium carbonate, titania, zirconia, zinc oxide,bismuth trioxide (Bi₂O₃), etc were weighed so that the respectivecompositions indicated in Table 1 were obtained. Thus raw materialbatches were prepared.

For the purposes of preventing bismuth trioxide from being reducedunnecessarily and refining glass, magnesium sulfate (MgSO₄) that was acommercially available reagent was used as a part of the MgO rawmaterial. In the composition containing Na₂O, sodium sulfate (Glauber'ssalt, Na₂SO₄) was used as a part of the Na₂O raw material. The contentof such sulfates was determined so that the mole ratio thereof tobismuth trioxide was at least 1/20.

Each batch thus prepared was put into an alumina crucible and was keptin an electric furnace at 1400° C. for four hours. Thereafter, themolten batch was poured on an iron plate to be cooled. The glass meltthat had been poured thereon was solidified in about ten seconds. Afterthis glass was kept in an electric furnace at 500° C. for 30 minutes,the power of the furnace was turned off and the glass then was cooledslowly to room temperature. Thus, respective glass samples (Samples 11to 18) were obtained.

Table 1 shows the characteristics determined with respect to those glasssamples. The respective glass samples were observed visually and werered or reddish brown. The light transmission spectra of all the glasssamples each had an optical absorption peak in the wavelength ranges of400 nm to 550 nm and 650 nm to 750 nm. FIG. 5 shows the lighttransmission spectrum of Sample 11 while FIG. 6 shows the opticalabsorption spectrum of Sample 11. The half-height width of the opticalabsorption peak at a wavelength of 490 nm shown in FIG. 6 is 100 nm. Allthe glass samples had an optical absorption peak whose half-height widthwas at least 30 nm.

Fluorescence whose wavelength was in the infrared region was observed inall the glass samples. FIG. 7 shows fluorescence spectra of Sample 11.It can be observed that fluorescence in a wide wavelength range,specifically 900 nm to 1400 nm, was obtained through each excitationcaused by irradiation of lights with wavelengths of 500 nm and 700 nm. Ahalf-height width of fluorescence of at least 150 μm was obtained in allthe glass samples including Sample 11. Furthermore, an emission lifetime(a lifetime of fluorescence) of at least 250 μs was obtained in all theglass samples.

In all the glass samples, it was observed that signal lights withwavelengths of 1064 nm and 1314 nm were amplified with excitation lighthaving a wavelength of 532 nm. As shown in Table 1, the wavelengths atwhich the fluorescence peak appeared in the fluorescence spectra are inthe wavelength region between 1064 nm and 1314 nm in all the glasssamples. Such glass samples allow optical amplification to be carriedout at least in a part of the above-mentioned wavelength region. Withconsideration given to fluorescence of the glass samples in a widewavelength range, the optical amplification can be carried out over arange of at least 250 nm.

The deformation points of those glasses were not shown in Table 1 butwere 750° C. or lower.

COMPARATIVE EXAMPLE 1

Glass raw materials were prepared by the same method as in Example 1 sothat the respective compositions indicated in Table 2 were obtained.Glass samples then were produced. In Sample 103, however, the batch thathad been prepared was put into a platinum crucible and was kept in anelectric furnace at 1450° C. for four hours. Thereafter, it was pouredon an iron plate to be cooled. After this glass was kept in an electricfurnace at 550° C. for 30 minutes, the power of the furnace was turnedoff and the glass then was cooled slowly to room temperature. Thus, theglass sample was obtained.

Using those glass samples, their characteristics were determined in thesame manner as in Example 1. The results are shown in Table 2.

Samples 101 and 102 had no gloss at their surfaces and had devitrifiedcompletely to the inside thereof. Sample 103 had a common soda-limeglass composition. It, however, was transparent and colorless and had nooptical absorption peak observed in the transmission spectrum thereof.Sample 103 did not emit light in the infrared region even when beingirradiated with light having a wavelength in the range of 400 nm to 850nm.

EXAMPLE 2 Phosphate Glass

In this example, glass compositions were obtained using three types ofproduction methods A to C.

Production Method A (Method Including Melting After a Heat Treatment)

Ammonium dihydrogen phosphate, alumina, lithium carbonate, sodiumcarbonate, potassium carbonate, magnesium oxide, calcium carbonate,strontium carbonate, barium carbonate, titania, zirconia, silica, zincoxide, bismuth trioxide, etc that were commercially available rawmaterials were weighed so that the respective compositions indicated inTable 3 were obtained. Thus raw material batches were prepared. Insteadof the above-mentioned ammonium salt, other salts or phosphoric acid maybe used as a phosphorus supply source.

As in Example 1, magnesium sulfate (MgSO₄) that was commerciallyavailable as a reagent also was used as a part of the MgO raw materialin this example. In the composition containing Na₂O, sodium sulfate(Glauber's salt, Na₂SO₄) was used as a part of the Na₂O raw material.The content of the sulfate was 0.5 mol % in terms of oxides thereof.

Each batch thus prepared was put into an alumina crucible. It was placedin an electric furnace and the temperature inside thereof then wasraised from room temperature to 1000° C. over four hours. It further waskept in the electric furnace at 1000° C. for four hours. This slowtemperature rise is effective in preventing the alumina crucible frombreaking. The carbonates and ammonium salt that are contained in thebatch are decomposed during the period of the temperature rise and thesubsequent heating. In this manner, when salts other than oxides havebeen decomposed beforehand, intense bubbling can be prevented fromoccurring in the melting process.

After the heat treatment, the batch that still was contained in thealumina crucible was moved into an electric furnace whose temperaturewas 1400° C. and then was kept for four hours to be melted. Thereafter,it was poured on an iron plate to be cooled. The glass melt that hadpoured thereon was solidified in about ten seconds. After this glass waskept in an electric furnace at 600° C. for 30 minutes, the power of thefurnace was turned off and the glass then was cooled slowly to roomtemperature. Thus, glass samples were obtained.

Production Method B (Method Including: Adding a Bi-Containing Batch to aBatch Free from Heat-Treated Bi; and Melting It)

The glass raw materials used herein were the same as those used inMethod A. However, the glass raw materials were divided into a firstbatch composed of the raw materials other than bismuth trioxide andmagnesium sulfate and a second batch containing the two raw materialsand were prepared so that the respective compositions indicated in Table3 were obtained. In this method, a predetermined amount of sulfate wascontained as a part of the raw materials as in Method A.

First, the first batch was heat-treated as in Method A. Next, this batchwas taken out of the alumina crucible and then was mixed with the secondbatch. Subsequently, the mixed batch was put into an alumina crucibleand then was kept at 1400° C. for four hours to be melted. Thereafter,the glass melt was poured on an iron plate to be cooled and then wascooled slowly using an electric furnace as in Method A. Thus, glasssamples were obtained. This method can prevent bismuth from beingreduced due to the decomposition of ammonium salt.

Production Method C (Method Including: Adding Bi to Glass Free from Bi;and Remelting It)

The same glass raw materials as those used in Method A were divided intoa first batch and a second batch as in Method B to be prepared.

As in Method B, the first batch was heat-treated as above. Subsequently,this batch that still was contained in the alumina crucible was movedinto an electric furnace whose temperature was 1400° C. and then waskept for two hours to be melted. Thereafter, this was poured on an ironplate to be solidified. This solid included bubbles but was colorlessand transparent glass.

This glass was pulverized and the second batch was added thereto, whichthen was mixed well together. This was put into an alumina crucible andwas kept in an electric furnace at 1400° C. for four hours to be melted.After this, the same procedure as in Method A was carried out. That is,the melt was poured on an iron plate to be cooled and then was cooledslowly using an electric furnace. Thus glass samples were obtained.

This method also can prevent bismuth from being reduced due to thedecomposition of ammonium salt. Furthermore, this method makes it easierto obtain a glass having excellent homogeneity that has less bubbles,striae, and coloring unevenness.

Glass samples (Samples 21 to 28) were obtained by any one of Methods Ato C. The characteristics determined with respect to these samples areindicated in Table 3. In this case, the transmittance denotes a valueobtained after subtracting the Fresnel reflection loss caused at thesurface of each glass sample.

All the glass samples were observed visually and as a result, were redor reddish brown. The light transmission spectra of all the glasssamples each had an optical absorption peak in the wavelength ranges of400 nm to 550 nm and 650 nm to 750 nm. FIG. 8 shows light transmissionspectra of Samples 21 to 24. Spectra indicating similar characteristicsto those of Samples 21 to 24 were obtained from the other samples.

Fluorescence in the infrared region was observed in all the glasssamples. FIG. 9 shows fluorescence spectra of Sample 21. In all theglass samples including Sample 21, the wavelength width of fluorescencewas at least 150 μm. Furthermore, in all the glass samples, an emissionlifetime (a lifetime of fluorescence) of at least 200 μs was obtainedwhen the excitation light had a wavelength of 450 nm while an emissionlifetime of at least 300 μs was obtained when the excitation light had awavelength of 700 nm.

In all the glass samples, it was observed that signal lights withwavelengths of 1064 nm and 1314 nm were amplified with excitation lightwith a wavelength of 532 nm. In all the glass samples produced inExample 2, the wavelengths at which the fluorescence peaks were obtainedalso were in the wavelength region between 1064 nm and 1314 nm.

The optical absorption peak whose half-height width was at least 30 nmwas observed in all the glass samples produced in Example 2. Thedeformation points were 750° C. or lower in all the glass samples.

EXAMPLE 3

An optical fiber sample was produced and optical amplificationcharacteristics thereof were determined. The optical fiber sample wasproduced so as to have a core diameter of 50 μm. In the optical fibersample, a glass having a composition of Sample 21 was used as a coreglass while a glass having a composition that was the same compositionas that of Sample 24 but was free from Bi₂O₃ was used as a clad glass.The optical fiber sample was cut into a length of 10 cm so as to havesections that were specular surfaces.

When intermittent irradiation of excitation light with a constantintensity was carried out with a chopper (omitted in FIG. 4) in aconstant cycle while signal light with a wavelength of 1314 nm wasallowed to enter the optical fiber sample, the intensity of the signallight increased during the irradiation of excitation light. FIG. 10shows the results of the measurement of variations in signal lightintensity that was carried out with an oscilloscope. It can be observedthat an amplification gain of 13.0 times (11 dB) was obtained at awavelength of 1314 nm.

COMPARATIVE EXAMPLE 2

Raw materials were prepared by the same method as in Example 1 so thatthe respective compositions indicated in Table 4 were obtained. Glasssamples then were produced.

In Comparative Example 201, however, the batch that had been preparedwas put into an alumina crucible and was kept at 1750° C. for fourhours. In Comparative Example 201, since it was not possible to pour theglass melt out of the crucible, the glass melt was cooled slowly whilebeing in the crucible. Thereafter, the glass sample was cut out. Theglass sample had been colored red. It, however, included numerousbubbles and striae and had an optical transmittance of only about 30% inthe wavelength range of 1000 nm to 1600 nm. In Comparative Example 202,a white opaque solidified substance was obtained but only a slight partthereof had been melted. In Comparative Example 203, the meltdevitrified during cooling after it was poured out.

Hereinafter, the reasons for the limitations on compositions aredescribed with reference to the results of Examples and ComparativeExamples.

Bismuth oxide is an essential element for allowing a glass compositionof the present invention to emit or amplify light. A preferable bismuthoxide is a bismuth trioxide (Bi₂O₃) or a bismuth pentoxide (Bi₂O₅). Anexcessively low content by percentage of bismuth oxide results in anexcessively low intensity of fluorescence in the infrared region that isprovided by bismuth oxide. On the other hand, an excessively highcontent by percentage thereof results in the optical absorption peaktending not to appear in the wavelength range of 450 nm to 550 nm in alight transmission spectrum and thereby the emission intensity decreasesin the infrared region. The content of bismuth oxide (in terms of Bi₂O₃)is preferably 0.01% to 5%, more preferably 0.01% to 3%, and particularlypreferably 0.1% to 3%.

A preferable example of the main component of the glass network formeris B₂O₃. An increase in content by percentage of B₂O₃ results in anincrease in emission intensity of a glass composition but results in anincrease in viscosity of a glass melt at the same time. When the contentby percentage of B₂O₃ exceeds 90%, it is difficult to produce a glasscomposition. On the other hand, an excessive decrease in content bypercentage of B₂O₃ results in a decrease in emission intensity of theglass composition in the infrared region and furthermore, allows theglass composition to tend to devitrify. When the content by percentageof B₂O₃ is less than 30%, no glass composition can be obtained.Accordingly, the content by percentage of B₂O₃ is preferably 30% to 90%,more preferably 34% to 75%, and particularly preferably 45% to 75%.

Another preferable example of the main component of the glass networkformer is P₂O₅. In order to prevent a glass composition fromdevitrifying and to obtain a homogeneous glass, the content bypercentage of P₂O₅ is preferably 50% to 80%, more preferably 60% to 75%.

Al₂O₃ is an essential component for allowing bismuth oxide to emitinfrared light in a glass composition. When the content by percentagethereof is less than 5%, this effect is not exhibited. On the otherhand, the emission intensity of the glass composition increases with anincrease in content by percentage of Al₂O₃. However, when the content bypercentage thereof exceeds 30%, the solubility of glass raw materialsdeteriorate and the glass composition tends to devitrify even if theglass raw materials have been melted completely. Accordingly, thecontent by percentage of Al₂O₃ is preferably 5% to 30%, furtherpreferably 10% to 30%, more preferably 10% to 25%, and particularlypreferably 5% to 25%.

It is preferable that divalent metal oxides MO (MO=MgO+CaO+SrO+BaO+ZnO)and univalent metal oxides R₂O (R₂O=Li₂O+Na₂O+K₂O) be added to vitrify acomposition. From this point of view, it is advantageous to add at least3% of MO+R₂O. Glass is homogenized more easily with an increase incontent by percentage of MO+R₂O. On the other hand, when the content bypercentage of MO+R₂O exceeds 40%, devitrification becomes very likely tooccur with an extremely high probability. Accordingly, the content bypercentage of RO+M₂O is preferably 3% to 40%, further preferably 5% to35%, more preferably 5% to 30%, and particularly preferably 10% to 30%.

It is advantageous that salt with high oxidizability such as sulfate(MSO₄, R₂SO₄), nitrate (M(NO₃)₂, RNO₃), etc. is used as a part of theraw materials of MO and R₂O. This is because a compound with highoxidizability is produced in a melting process and can prevent bismuthfrom being reduced. When the bismuth is prevented from being reduced,the container to be used for melting such as a platinum or platinumalloy crucible also can be prevented from being eroded. A preferableamount of sulfate and nitrate that is expressed in a mole ratio is atleast 1/20 of bismuth oxide.

MgO is an important glass network modifier. MgO improves meltability ofa raw material batch. However, an excessively high content by percentageof MgO causes a glass composition to exhibit a dark brown color, theoptical absorption peak in the wavelength range of 450 nm to 550 nm todecrease, and accordingly the emission intensity to decrease rapidly. Anexcessively high content by percentage of MgO results in excessively lowviscosity of a glass melt to cause devitrification readily. The contentby percentage of MgO is preferably 0% to 40%, further preferably 0.1% to35%, more preferably 0.1% to 30%, and particularly preferably 0.5% to30%.

Like MgO, CaO improves the meltability of a raw material batch and issuperior to MgO in characteristic of improving the devitrificationresistance of glass. As in the case of MgO, however, when the content bypercentage of CaO is excessively high, glass exhibits a dark brown colorand thereby has a decreased emission intensity. Accordingly, the contentby percentage of CaO is preferably 0% to 30%, further preferably 0% to20%, more preferably 0% to 18%, and particularly preferably 0% to 10%.

Like MgO and CaO, SrO also improves the meltability of a raw materialbatch. Even a small amount (for instance, 0.1% or more) of SrO improvesthe devitrification resistance of glass considerably. SrO, however, hasa strong effect of rapidly decreasing the intensity of fluorescence thatis provided by bismuth. Accordingly, the content by percentage of SrO ispreferably 0% to 15%, more preferably 0% to 5%.

Like MgO and CaO, BaO also improves the meltability of a raw materialbatch. BaO has a higher effect of improving the refractive index ascompared to other divalent metal oxides. Since the increase inrefractive index results in improvement in luster of a glass surface,the development of red or reddish brown color also is improved. Hence,it is advantageous that for instance, at least 0.1% of BaO is added.BaO, however, has a strong effect of rapidly decreasing emissionintensity. Accordingly, the content by percentage of BaO is preferably0% to 15%, more preferably 0% to 5%.

ZnO also improves the meltability of a raw material batch. ZnO has agreater effect of allowing the color of glass to develop into red orreddish brown as compared to CaO, SrO, and BaO. ZnO also is excellent inthe effect of increasing the refractive index of glass as compared toMgO. With consideration given to this, a small amount (for instance,0.1% or more) of ZnO may be added. As in the case of MgO, however, whenthe content by percentage of ZnO is excessively high, glass exhibits adark brown color and thereby has a decreased emission intensity. Whenthe content by percentage of ZnO is excessively high, glass may sufferphase separation to become cloudy and thereby transparent glass may notbe obtained. Accordingly, the content by percentage of ZnO is preferably0% to 25%, further preferably 0% to 15%, and more preferably 0% to 10%.

Li₂O is an important glass network modifier. Li₂O decreases the meltingtemperature to improve meltability and also improves the refractiveindex of glass. An addition of a suitable amount of Li₂O improvesoptical absorption to increase the emission intensity. It therefore isadvantageous to add at least 0.1% of Li₂O. As in the case of MgO,however, when the content by percentage of Li₂O is excessively high,glass exhibits a dark brown color and thereby has a decreased emissionintensity. A still higher content by percentage of Li₂O results indecreased viscosity of a glass melt and thereby devitrification tends tooccur. The content by percentage of Li₂O is preferably 0% to 30%, morepreferably 0% to 15%, and particularly preferably 0% to 12%.

Na₂O lowers the melting temperature as well as the liquidus temperatureand thereby prevents glass from devitrifying. Na₂O, however, has astrong effect of weakening fluorescence by making the glass dark brown.Accordingly, the content by percentage of Na₂O is preferably 0% to 15%,more preferably 0% to 5%.

K₂O lowers the liquidus temperature and thereby prevents glass fromdevitrifying. K₂O, however, weakens fluorescence of glass in theinfrared region even when a small amount thereof is added. Accordingly,the content by percentage of K₂O is preferably 0% to 5%, more preferably0% to 2%.

TiO₂ increases the refractive index of glass and promotes fluorescence.BaO has a strong effect of decreasing the emission intensity while TiO₂has an effect of improving the emission intensity. TiO₂, however, has aneffect of making glass cloudy. Accordingly, the content by percentage ofTiO₂ is preferably 0% to 10%, more preferably 0% to 5%.

Like TiO₂, ZrO₂ improves the refractive index of glass and promotesinfrared fluorescence. ZrO₂, however, has an effect of acceleratingcrystallization of glass and increasing the density of glass.Accordingly, in order to prevent the devitrification from occurring andthe density from increasing, the content by percentage of ZrO₂ ispreferably 0% to 5%, more preferably 0% to 3%.

The glass composition of the present invention may include a pluralityof glass network formers and may contain, for instance, SiO₂. Anaddition of SiO₂ provides an effect of preventing devitrification fromoccurring. An excessively high content by percentage of SiO₂, however,results in an excessively high viscosity of a glass melt and therebyhinders the composition from being homogenized. The content bypercentage of SiO₂ is preferably 0% to 20%.

Furthermore, for the purposes of, for instance, controlling therefractive index, controlling temperature viscosity characteristics, andinhibiting devitrification, the glass compositions of the presentinvention may contain Y₂O₃, La₂O₃, Ta₂O₅, Nb₂O₅ and In₂O₃, preferablywith the total content by percentage thereof being 5% or less, inaddition to the above-mentioned components.

Moreover, for the purposes of, for instance, allowing glass to be clearwhen it is melted and preventing bismuth from being reduced, the glasscomposition of the present invention may include As₂O₃, Sb₂O₃, SO₃,SnO₂, Fe₂O₃, Cl and F, preferably with the total content by percentagethereof being 1% or less.

Components other than those described above may be introduced, as traceamounts of impurities, into glass raw materials. However, when the totalcontent by percentage of such impurities is less than 1%, the ultimateeffect on the physical properties of the glass composition is small andtherefore does not cause any substantial problems.

It is not necessary for the glass compositions of the present inventionto contain Nd, Er, Pr, Ni, and Cr in order to exhibit a fluorescencefunction or an optical amplification function. Accordingly, the glasscomposition may be substantially free from those elements. In thiscontext, the expression “substantially free” denotes that the contentsby percentage thereof are less than 1%, preferably less than 0.1% interms of oxides thereof that have the highest stability in glass.

The glass compositions of the present invention can be used in the1310-nm range and at 1064 nm. The 1310-nm range is one of the wavelengthranges that are used in optical communications mainly while 1064 nm isthe emission wavelength of a Nd-YAG laser. The present invention canprovide a new optical amplification medium that works in the wavelengthrange of 1100 nm to 1300 nm, for which no suitable optical amplificationmaterial has been reported. The glass compositions of the presentinvention can provide broad fluorescence spectra over 900 nm to 1400 nmin at least preferable embodiments thereof. The use of them makes itpossible to provide light amplifiers that operate in that widewavelength range. TABLE 1 Sample 11 12 13 14 15 16 17 18 Composition(mol%) B₂O₃ 59.7 59.7 59.7 59.7 59.7 59.7 59.7 59.7 Al₂O₃ 24.9 22.4 19.924.9 24.9 24.9 24.9 24.9 Li₂O 0 0 0 0 0 0 3.0 0 Na₂O 0 0 0 0 0 0 0 1.0K₂O 0 0 0 0 0 0 0 1.0 MgO 14.9 17.4 19.9 5.0 9.9 5.9 10.9 11.9 CaO 0 0 09.9 0 0 0 0 SrO 0 0 0 0 5 0 0 0 BaO 0 0 0 0 0 0 1.0 0 TiO₂ 0 0 0 0 0 1.00 0 ZrO₂ 0 0 0 0 0 0 0 1.0 ZnO 0 0 0 0 0 8.0 0 0 Bi₂O₃ 0.5 0.5 0.5 0.50.5 0.5 0.5 0.5 MO + R₂O 14.9 17.4 19.9 14.9 14.9 13.9 14.9 13.9Presence of Optical Absorption Peak 400 nm to 550 nm Yes Yes Yes Yes YesYes Yes Yes 650 nm to 750 nm Yes Yes Yes Yes Yes Yes Yes YesFluorescence Spectrum obtained through Excitation at 500 nm Wavelengthof Peak 1096 1107 1112 1104 1109 1099 1117 1105 Fluorescence (nm)Half-Height Width of 200 197 195 199 200 198 195 195 Fluorescence (nm)Fluorescence Spectrum obtained through Excitation at 700 nm Wavelengthof Peak 1080 1087 1094 1085 1091 1082 1097 1086 Fluorescence (nm)Half-Height Width of 194 190 186 192 192 191 187 188 Fluorescence (nm)Lifetime of Fluorescence Excitation at 500 nm and 304 295 288 270 285284 291 283 Measurement at 1140 nm (μs) Linear Expansion Coefficient 6066 68 65 62 65 64 65 (10⁻⁷° C.) Glass Transition Point (° C.) 644 633612 598 631 644 609 603 Deformation Point (° C.) 686 662 646 641 668 691651 647

TABLE 3 Sample Composition (mol %) 21 22 23 24 25 26 27 28 P₂O₅ 67.067.3 67.3 64.8 67.3 69.3 74.3 55.2 Al₂O₃ 22.3 22.4 22.4 19.9 22.4 16.49.7 19.8 Li₂O 9.9 0.0 0.0 0 0 5 8.0 0 Na₂O 0 0 0 0 0 1 2 0 K₂O 0 0 0 0 00 0 1 MgO 0.5 10.0 10.0 15.0 0.5 1.7 0.9 13.0 CaO 0 0 0 0 9.5 3 0 0.0SrO 0 0 0 0 0 0 2 0.0 BaO 0 0 0 0 0 0 0 3.0 TiO₂ 0 0.0 0 0 0 2 0 0 ZrO₂0.0 0.0 0.0 0 0 1 0.0 0 ZnO 0 0 0 0 0 0.0 3 0 SiO₂ 0 0 0 0 0 0 0 5.0Bi₂O₃ 0.3 0.3 0.3 0.3 0.3 0.6 0.1 3.0 MO + R₂O 10.4 10.0 10.0 100 1010.7 15.9 17.0 Glass Production Method B B A B B C A A Presence ofOptical Absorption Peak 450 nm to 550 nm Yes Yes Yes Yes Yes Yes Yes Yes650 nm to 750 nm Yes Yes Yes Yes Yes Yes Yes Yes Transmittance of 3-mmThick Sample (%) Minimum Value at 1000 nm 89 87 80 89 92 85 80 82 to1600 nm Fluorescence Spectrum obtained through Excitation at 450 nmWavelength of Peak 1115 1180 1182 1115 1175 1120 1130 1140 Fluorescence(nm) Half-Height Width of 236 237 258 236 230 220 240 230 Fluorescence(nm) Fluorescence Spectrum obtained through Excitation at 700 nmWavelength of Peak 1122 1132 1132 1122 1130 1120 1130 1120 Fluorescence(nm) Half-Height Width of 177 189 198 177 170 180 170 180 Fluorescence(nm) Fluorescence Spectrum obtained through Excitation at 833 nmWavelength of Peak 1204 1253 1263 1204 1250 1240 1250 1240 Fluorescence(nm) Half-Height Width of 332 323 306 332 300 310 300 310 Fluorescence(nm) Lifetime of Fluorescence (μs) Excitation at 450 nm and 320 343 289320 310 295 275 290 Measurement at 1140 nm Excitation at 700 nm and 487493 408 487 450 430 410 420 Measurement at 1120 nm Excitation at 833 nmand 167 158 142 167 160 150 140 160 Measurement at 1250 nm RefractiveIndex 1.520 1.504 1.513 1.506 1.514 1.518 1.512 1.519 Abbe Number 65 7187 65 70 66 68 69 Linear Expansion Coefficient 66 55 56 62 63 68 63 60(10⁻⁷ ° C.) Glass Transition Point (° C.) 528 646 648 605 663 595 638643 Deformation Point (° C.) 584 703 702 659 708 649 692 688

TABLE 2 Sample Composition(mol %) 101 102 103 SiO₂ 0 0 70.4 B₂O₃ 24.844.9 0 Al₂O₃ 29.8 34.9 2.3 Li₂O 44.6 10.0 0 Na₂O 0 0 13 K₂O 0 0 0 MgO0.5 10.0 6 CaO 0 0 8 SrO 0 0 0 BaO 0 0 0 TiO₂ 0 0 0 ZrO₂ 0 0 0 Bi₂O₃ 0.30.3 0.3 Glass Devitrified Devitrified Vitrified Color Tone of Glass — —Colorless and Transparent Optical Absorption Peak — — None

TABLE 4 Sample Composition(mol %) 201 202 203 P₂O₅ 0 54.9 51.8 Al₂O₃ 2.244.8 2.0 Li₂O 0 0 0 Na₂O 0 0 0 K₂O 0 0 0 MgO 0 0 45.9 CaO 0 0 0 SrO 0 00 BaO 0 0 0 TiO₂ 0 0 0 ZrO₂ 0 0 0 ZnO 0 0 0 SiO2 97.5 0 0 Bi₂O₃ 0.3 0.30.3 MO + R₂O 0 0 45.9 State of Glass Vitrified Not Devitrified MeltableColor Tone of Glass Red White Dark Brown Optical Absorption Peak 400 nmto 550 nm Present — — 657 nm to 750 nm Present — — Transmittance of 3-mm30 — — Thick Sample (%) Minimum Value at 1000 nm to 1600 nm

1. A glass composition comprising: a bismuth oxide; an aluminum oxide;and a glass network former, wherein the glass network former includes anoxide other than a silicon oxide as its main component, and the glasscomposition emits fluorescence in an infrared wavelength region throughirradiation of excitation light, with bismuth contained in the bismuthoxide functioning as a fluorescent source.
 2. The glass compositionaccording to claim 1, wherein the main component of the glass networkformer is a phosphorus pentoxide, a boron oxide, a germanium oxide, or atellurium dioxide.
 3. The glass composition according to claim 1, havingan optical absorption peak in a wavelength range of 400 nm to 900 nm. 4.The glass composition according to claim 1, wherein a wavelength atwhich the maximum intensity of the fluorescence that is emitted throughthe irradiation of excitation light having a wavelength in a range of400 nm to 900 nm is obtained is in a range of 900 nm to 1600 nm.
 5. Theglass composition according to claim 4, wherein a half-height width withrespect to the wavelength of the fluorescence is at least 150 nm.
 6. Theglass composition according to claim 1, providing a gain in signal lightamplification in at least a part of a wavelength range of 900 nm to 1600nm through the irradiation of excitation light.
 7. The glass compositionaccording to claim 1, further comprising a univalent or divalent metaloxide.
 8. The glass composition according to claim 7, wherein thedivalent metal oxide is at least one selected from the group consistingof MgO, CaO, SrO, BaO, and ZnO.
 9. The glass composition according toclaim 7, wherein the univalent metal oxide is at least one selected fromthe group consisting of Li₂O, Na₂O, and K₂O.
 10. The glass compositionaccording to claim 7, wherein the content of the metal oxide that isunivalent or divalent is in a range of 3 mol % to 40 mol %.
 11. Theglass composition according to claim 1, wherein the content of thebismuth oxide is in a range of 0.01 mol % to 15 mol % in terms of Bi₂O₃.12. The glass composition according to claim 11, wherein the content ofthe bismuth oxide is in a range of 0.01 mol % to 5 mol % in terms ofBi₂O₃.
 13. The glass composition according to claim 1, wherein thecontent of the aluminum oxide is in a range of 5 mol % to 30 mol %. 14.The glass composition according to claim 1, wherein the content of themain component of the glass network former is in a range of 30 mol % to90 mol %.
 15. The glass composition according to claim 2, comprising thefollowing components, indicated by mol %: 30 to 90 B₂O₃; 5 to 30 Al₂O₃;0 to 30 Li₂O; 0 to 15 Na₂O; 0 to 5 K₂O; 0 to 40 MgO; 0 to 30 CaO; 0 to 5SrO; 0 to 5 BaO; 0 to 25 ZnO; 0 to 10 TiO₂; and 0 to 5 ZrO₂, wherein thetotal of MgO+CaO+SrO+BaO+ZnO+Li₂O+Na₂O+K₂O is in a range of 3 mol % to40 mol %, and the content of the bismuth oxide is in a range of 0.01 mol% to 15 mol % in terms of Bi₂O₃.
 16. The glass composition according toclaim 2, comprising the following components, indicated by mol %: 50 to80 P₂O₅; 5 to 30 Al₂O₃; 0 to 30 Li₂O; 0 to 15 Na₂O; 0 to 5 K₂O; 0 to 40MgO; 0 to 30 CaO; 0 to 15 SrO; 0 to 15 BaO; 0 to 15 ZnO; 0 to 10 TiO₂; 0to 5 ZrO₂; and 0 to 20 SiO₂, wherein the total ofMgO+CaO+SrO+BaO+ZnO+Li₂O+Na₂O+K₂O is in a range of 3 mol % to 40 mol %,and the content of the bismuth oxide is in a range of 0.01 mol % to 15mol % in terms of Bi₂O₃.
 17. An optical fiber comprising a glasscomposition according to claim
 1. 18. A light amplifier comprising aglass composition according to claim
 1. 19. A method of manufacturing aglass composition according to claim 1, comprising: melting a rawmaterial of the glass composition; and cooling the raw material that hasbeen melted, wherein the method further comprises, before melting theraw material, a heat treatment step in which a first material thatcontains ammonium salt and that is at least a part of the raw materialis maintained at a temperature at which at least the ammonium saltdecomposes.
 20. The method of manufacturing a glass compositionaccording to claim 19, further comprising, after the heat treatment stepbut before the melting step, a step of mixing the first material with asecond material that includes a raw material of bismuth oxide or abismuth oxide.
 21. A method of amplifying signal light by allowingexcitation light and signal light to enter a glass composition accordingto claim 1 to amplify the signal light.